The present invention relates to a receiver for use in a mobile cellular phone operating with a W-CDMA protocol using FDD. Particularly, the invention relates to the removal of a SAW filter inserted between a low noise amplifier and a mixer in a receiving stage.
A W-CDMA (Wideband Code Division Multiple Access) protocol is a protocol for wireless connection which is used for so-called third generation cellular mobile phones. The W-CDMA is characterized in that signals diffused over a wide band are received and subjected to back diffusion to acquire significant signals. The W-CDMA protocol is performed using an FDD (Frequency Division Duplex) method and a TDD (Time Division Duplex) method. At present, the FDD method is generally used.
The W-CDMA protocol essentially requires a high-performance filter that receives signals diffused over a wide band, while cutting off radio waves of other frequency bands. In addition, parts which are used in a cellular mobile phone or the like are demanded to be smaller and lighter. To satisfy these conditions, a SAW (Surface Acoustic Wave) filter is generally used.
However, the SAW filter is a part and there is a cost for procuring it. A space for installing it must be allocated, though this space is just small. Therefore, if a filterless receiver can be designed, it is most favorable.
Because of such demand, curtailing the number of filters is lately becoming a general practice by using a zero IF (direct conversion) method when modulating an RF frequency band to a baseband frequency band. Such configuration is adopted in a receiver disclosed in Japanese Unexamined Patent Publication No. 2006-115345(Patent Document 1).
[Patent Document 1]
    Japanese Unexamined Patent Publication No. 2006-115345